The present invention relates generally to integrated circuits, and more specifically to a MOS bandgap reference circuit.
Bandgap reference circuits are used in a variety of integrated circuit devices as a means for sensing changes in the voltage or current level of a device, so that appropriate changes in the operation of the integrated circuit device may be made. In many memory integrated circuit devices, it is desirable to ensure that data stored in the memory is retained and not lost or corrupted upon a loss of power to the device. For example, static random access (SRAM) memory devices referred to as xe2x80x9czero powerxe2x80x9d devices must be able to sense and respond to changes in the supply voltage. In a zero power SRAM, the data content of the SRAM is protected when the power supply voltage supplied to the SRAM drops below some predetermined voltage level. Typically, the data content of the zero power SRAM is protected by switching from a primary power source to a secondary power source when the power supply voltage to the zero power SRAM falls below the predetermined voltage level.
In order to switch from the primary power source to a secondary power source, it is necessary to be able to sense the voltage level of the primary power source and automatically switch to the secondary power source when appropriate. A bandgap reference circuit is one effective means to determine when it is necessary to switch from the primary power source to the secondary power source of a zero power SRAM. U.S. Pat. No. 4,451,742 issued May 29, 1984 to Aswell describes switching from a primary to a secondary power source and is herein incorporated by reference. However, typical bandgap reference circuits require a large number of bipolar devices which of course consume a large portion of the integrated circuit area of the SRAM. Therefore, because bandgap reference circuits may be effectively used in a zero power SRAM to determine the switching point, it would be desirable to be able to use an improved bandgap reference circuit which has fewer bipolar devices and thus consumes less area and power than bandgap reference circuits currently available.
It would be advantageous in the art to utilize a bandgap reference circuit which has fewer bipolar junction transistors than the prior art bandgap reference circuit.
It would further be advantageous to the art to utilize a bandgap reference circuit which provides scaling of current through bipolar junction transistors.
Therefore, according to the present invention, a bandgap reference circuit which utilizes a minimum number of bipolar devices and current mirror scaling devices generates a bangap reference voltage. The bandgap voltage generated by the bandgap reference circuit is a function of a plurality of sized current mirror devices, the ratio of a first resistor to a second resistor, and the number and relative sizing of bipolar junction transistors used. The bandgap reference circuit generates a bandgap reference voltage which is suitable for use in a variety of integrated circuit devices, such as a zero power static random access memory (SRAM). If used in a zero power SRAM application, the bandgap reference voltage may be utilized to determine when the primary power source of the zero power SRAM has fallen below a predetermined voltage level and a secondary power source must be substituted for the primary power source.